Spectrometers have been used to determine the concentration of fluids and the concentration of components found in mixtures of fluids. For example, the absorption characteristics of a gas at specific radiation wavelengths can be used to identify and quantify the concentration of the gas. This process is best defined by the Beer-Lambert law, which states that the transmittance of radiation through a gas that absorbs radiation is decreased exponentially and directly proportional to the length of the radiation path and the concentration of the gas. This relationship is shown in Equation 1: EQU T =I/I.sub.o =e.sup.-acl ( 1)
where:
T =transmittance of the radiation through the gas PA1 I.sub.o =intensity of the radiation entering the gas PA1 I =intensity of the radiation leaving the gas PA1 a =molar absorptivity PA1 c =concentration of the gas PA1 l =distance the radiation beam travels through the gas
The molar absorptivity, a, is dependent upon the wavelength of the radiation and upon the characteristics of the gas. The molar absorptivity indicates the degree to which a molecule will absorb radiation at a given wavelength. This can be determined by calibration on a given spectrometer. The molar absorptivity is a constant. Thus, once it is determined for a given spectrometer, it should theoretically not have to be determined again for that spectrometer.
In most spectrometers, it is not practical to measure the radiation intensities I and I.sub.o simultaneously. Therefore, the initial intensity, I.sub.o, is determined by a measurement made during a calibration step in which all radiation absorbing gases are purged from the sample cell. However, purging is costly, both in terms of added equipment cost and added labor costs. Additionally, purging is often impractical when a sample cell is not employed, i.e. during in situ measurements. The transmittance of the zero gas is by definition 100%. The spectrum obtained is the reference spectrum with which subsequent spectra are compared to determine the transmittance.
In an alternative method, the radiation beam path can be alternated between a reference path and the sample path, as disclosed in U.S. Pat. No. 4,158,505 by Mathisen et al. issued June 19, 1979. However, as can be seen from this patent, complicated mechanisms are required in order to provide for the switching of the beam between the reference and sample paths. Additionally, errors can be introduced if the reference and sample paths are not identical.
If more than one gas absorbs at the wavelength of interest, Beer's law dictates that the absorbance of a mixture is the sum of the absorbances of all the components of the mixture U.S. Pat. No. 3,893,770 by Takami et al. issued July 8, 1975, describes an apparatus for analyzing a plurality of mixed gases. The disclosed analyzer can measure component gases e.g. nitrogen dioxide (NO.sub.2), sulfur dioxide (SO.sub.2) and nitric oxide (NO), present in flue gases. It relies on detection of absorption spectra by simultaneous measurement of the intensity of radiation at several different discrete wavelengths. Interferences between spectra of different gases are compensated for by means of appropriate function generation and arithmetic units in the system's output circuitry. The interferences must be "irreversible" in order for the unit to operate. Additionally, a means must still be provided in order to obtain the initial reference intensity, I.sub.o, in order to calculate the relative absorbances.
Another difficulty encountered in using an instrument of the type disclosed in U.S. Pat. No. 3,893,770 for characterizing components in flue gas is due to the environment in which the instrument has to operate. Since the instrument is measuring only a single wavelength of the spectrum for each gaseous component, any misalignment due to vibration or temperature or pressure gradients will produce inaccuracies in the measurement. In addition, the accuracy of the absorbance calculation is dependent upon the stability of the reference measurement. Since the reference measurement cannot be made continuously, any change in the output of the radiation source or distortion in the optics will produce a source of error.
In an article entitled "Development of Low Level NH.sub.3 Measuring Method" by Nakabayashi et al., a method for measuring NH.sub.3 at low concentrations is disclosed. In this method, the need for obtaining a reference intensity level, I.sub.o, is eliminated. However, wavelength modulation of a certain angular frequency must be applied at a wavelength corresponding to an absorption peak. This requires the use of an oscillating mirror or slit. Such a moving mechanism in a spectrometer could be a source of error if the oscillation frequency were to deviate or the orientation of the mirror or slit were to become misaligned.
Another problem with the Nakabayashi device is that it requires frequent calibration. For example, a method for dealing with interference from SO.sub.2 is disclosed, however, weekly calibration of the instrument is required. In addition, a zero point calibration must be carried out every six hours. Such frequent calibrations are obviously undesirable.
The Nakabayashi device is also very sensitive to temperature shifts and, therefore, must be housed in a constant temperature chamber and heated to a temperature of 43.degree. C. Additionally, the sample path must be heated to a temperature of 300.degree. C. in order to avoid the deposition of acid ammonium sulfate, which occurs below 250.degree. C.
Therefore, it would be advantageous to have a spectrometer which does not require frequent mechanical calibrations.
It would also be advantageous to have a spectrometer which can be used to measure the concentration of gases in the presence of other interfering gases.
Further, it would be advantageous to be able to measure the concentration of gases continuously.
Additionally, it would be advantageous to measure the concentration of gases without having to periodically obtain a reference, or zero gas, measurement.
Furthermore, it would be advantageous to have a spectrometer with few moving parts.
Also, it would be advantageous to have a spectrometer which could be used both in situ and in an extractive environment.
Further, it would be advantageous to have a spectrometer in which variations in the intensity of the radiation source did not affect the accuracy of measurements.